Ultrasonic testing (UT) is a widely used non-destructive testing technique to perform quality checks on various materials in order to detect internal flaws or to perform quality control. Typically, a UT probe is a transducer that emits an ultrasonic pulse into the object under inspection, and detects a reflected pulse. In the absence of a defect in the path of the pulse, the pulse will be reflected from the far boundary of the object, and will have a characteristic amplitude. A defect in the path of the pulse will result in a pulse reflected by the defect. The reflected pulse returns to the probe where it is detected.
UT inspection works very well for an object with relatively flat surfaces. Not all objects are suited to this type of inspection, for example it is more difficult to achieve a high level of accuracy when inspecting objects that have rounded surfaces.
However, it can be necessary to ensure the quality of a spherical object such as a steel bearing ball. A ball bearing can only function properly when the surfaces of the race and the surfaces of each bearing ball are smooth and free of defects. Small bearing balls can be manufactured relatively easily to a high standard. Larger bearing balls are more difficult to manufacture to the required standards since their larger dimensions increase the likelihood of surface defects. Therefore, much effort is invested in the inspection of such bearing balls, and an inspection procedure with a high probability of detection (PoD) rate is necessary to ensure that a flawed bearing ball does not make it through quality control. A visual inspection is slow and limited to the detection of evident defects such as pits or cracks in the outer surface. Such bearing balls would be discarded. However, a disadvantage of visual inspection is that a potentially problematic internal defect may escape detection. An internal defect located close to the surface of a bearing ball may later open up. An open defect can subsequently lead to seizure of the bearing and failure of the machine. In the case of large machine such as a wind turbine generator weighing several tons, failure of the ball bearing and subsequent repair can be very costly. Therefore, inspection of the components of such a large ball bearing may involve other techniques such as eddy current testing or UT inspection. However, when the known techniques of UT inspection are applied to a spherical body, the behavior of the pulses in the spherical body makes it difficult to interpret the reflections, so that a reliable inspection is not possible. Furthermore, the known techniques are not suited to detecting defects or flaws at or near the surface of a sphere.